1. Field of the Invention
The present invention relates to an electromagnetic wave sensor that selectively detects a specific frequency band of electromagnetic waves, such as sub-millimeter waves and millimeter waves, an imaging element and an imaging device.
2. Related Art
Relative to infrared light having wavelengths of several micrometers to tens of micrometers, millimeter waves (wavelengths of several mm to 10 mm) and sub-millimeter waves (wavelengths of about 30 μm to 1 mm) having longer wavelengths, which is also referred to as terahertz light in recent years, have attracted attention as electromagnetic waves that provide new modes of sensing and imaging for their characteristics, such as high penetrative properties and noninvasiveness to substances as well as distinct spectral characteristics depending on materials or conditions, and have been technically matured by recent development in lasers or optoelectronics.
Means for detecting electromagnetic waves of this band include a method for indirectly detecting desired electromagnetic waves by combining an ultra-short pulse laser with nonlinear crystals or an optical switch; a method for electrically detecting electromagnetic waves using a semiconductor device, such as an ultrahigh-frequency diode; a method using a quantum detecting element, such as a superconductive element and quantum-dot element; and a method using a thermal detecting element, such as a bolometer and pyroelectric element.
Although the method using an ultra-short pulse laser is superior for spectroscopy because spectral information on the subject of measurement can be easily obtained, it requires the entire system including the laser, the optical system and the subject of measurement to be placed in a well-controlled atmosphere, and suffers from many limitations, such as requirement of equipment which is expensive as a whole, to find application in the industry.
Detection using an ultrahigh-frequency diode has been realized of only up to several hundred gigahertzes at present, and has technical difficulties for expansion to higher frequency side.
Although use of the quantum detecting element is an excellent method for detecting only electromagnetic waves of frequencies corresponding to specific inter-level transition, and is capable of high-speed detection at high-sensitivity, the thermal excitation of carriers must be suppressed and the element must be cooled to an extremely low temperature for detecting electromagnetic waves whose photon energy is extremely small, such as millimeter waves and sub-millimeter waves, at high-sensitivity. Therefore, the quantum detecting element requires a liquid coolant, such as liquefied helium, and the fabrication of a practical system is difficult.
The thermal detecting element detects temperature change resulted from heat generated by converting electromagnetic waves using certain means, as the change in the electrical characteristics of the element. Although the thermal detecting element is slower and less sensitive than the quantum detecting element, it has generally flat sensitivity characteristics over a broad band, and temperature environment for the sensor can advantageously be stabilized to simplify cooling requirement by introducing a heat-insulating structure or the like to support the element in the air. Therefore, the thermal detecting element has been put into practical use as an infrared image sensor in the infrared wavelength band, and as a radio telescope in the millimeter wave band. The detection wavelength band of the thermal detecting element is designed by optimizing the materials of the heating element to optically absorb electromagnetic waves, or by adding a structure that can be resonated at a specific wavelength, such as an antenna and a waveguide, to the heating element. In an example wherein a thermal detecting element is applied to an infrared image sensor, vanadium oxide, polysilicon, amorphous silicon, germanium, titanium or the like is used as the material for the heating element. A bolometer is disclosed, for example, in U.S. Pat. No. 6,441,368.
In the band of sub-millimeter waves or millimeter waves, which have longer wavelengths than infrared light, it is difficult to design the material that efficiently absorbs only electromagnetic waves having a specific frequency. Since the thermal detecting element detects change in the temperature of the heating element as signals accordingly, there is a problem wherein the thermal detecting element is easily affected by electromagnetic waves having wavelengths other than the wavelength in the frequency band to be detected.
At normal temperatures, naturally radiated light whose peak is infrared light in the 10 μm band is radiated from a subject of measurement and the surroundings, and the intensity of the sub-millimeter wave band is about one-thousandth lower than the infrared band. Therefore, an important technical challenge in the selective detection of millimeter waves or sub-millimeter waves using a thermal detecting element, whose wavelength selectivity is not fundamentally high, is to reduce the effect of infrared light contained in naturally radiated light. To detect millimeter waves or sub-millimeter waves, a thermal detecting element wherein, for example, a dipole antenna, whose size is a half the wavelength λ to be detected, is joined to the heating element is available. There is a problem that, when naturally radiated light of a normal temperature is irradiated onto the thermal detecting element, the temperature of the heating element is elevated by direct irradiation or indirect irradiation by reflection from the surroundings or the like of infrared light, which is a major component of naturally radiated light. Since, with such an antenna, higher harmonics having wavelengths of an integral fraction of the wavelength λ of electromagnetic waves is also detected in a certain efficiency in addition to electromagnetic waves having a wavelength of λ, and the higher harmonics of sub-millimeter waves fall within the infrared region, the infrared light, which is a major component of naturally radiated light, may pass through the antenna and heat the heating element, and it is difficult to selectively detect only a specific frequency band of millimeter waves or sub-millimeter waves at high sensitivity.
To reduce the effect of infrared light contained in the naturally radiated light, a method can be used wherein strong monochromatic light having millimeter waves or sub-millimeter waves is radiated to a substance, and the reflected, scattered or transmitted light is detected. However, in this band, only large-scale research equipment, such as a methanol laser and a free-electron laser, is at the present available as a high-output light source that can ignore the effect of naturally radiated light, and it is difficult to apply such a light source to practical systems. In another method, an optical filter that blocks infrared light but transmits millimeter waves and sub-millimeter waves is placed in front of the detecting element. For example, in the case of a radio telescope, the element itself is cooled to an extremely low temperature in addition to the wavelength selecting structure, such as an antenna and a wave guide, and an optical filter is used for blocking naturally radiated light from the exterior to detect imperceptible millimeter waves and sub-millimeter waves coming from space. However, such large-scale equipment configuration cannot provide a simple and inexpensive detecting device of millimeter waves and sub-millimeter waves.